The continual demand for enhanced integrated circuit performance has resulted in, among other things, a dramatic reduction of semiconductor device geometries, and continual efforts to optimize the performance of every sub-structure within a semiconductor device. A number of improvements and innovations in fabrication processes, material composition, and layout of the active circuit levels of semiconductor devices have resulted in very high-density circuit designs. Increasingly dense circuit design has, consequently, greatly improved a number of performance issues—such as minimizing signal propagation delays through the active circuit levels of semiconductor devices.
Only recently, however, have other layers and structures within semiconductor devices received such scrutiny and been the subject of optimization efforts. For many years, most semiconductor devices utilized “back-end” (e.g., metal interconnect and dielectric) layers based on very mature aluminum (Al) and silicon dioxide (SiO2) technology. With the improvements in the active circuit levels, it was not uncommon for 50% or more of propagation delays to occur in the back-end layers, in semiconductor devices utilizing such mature back-end technologies. As a result, a large segment of semiconductor manufacturing is transitioning from the mature Al-based back-end materials and technologies to new, alternative materials and technologies. With that transition a number of new and unexpected challenges and problems arise—some of which are counter-intuitive based on an understanding of the mature back-end materials and technologies.